1. Field of the Invention
The present invention relates to a method for producing an isocyanurate-free uretidione dimer of 3-isocyanato-methyl-3,5,5-trimethylcyclohexylisocyanate (isophorone diisocyanate, IPDI) as well as the uretidione dimer produced according to this method.
2. Description of the Prior Art
Aromatic substituted uretidiones and their preparation by dimerization of aromatic isocyanates using tertiary amines or phosphines as catalysts have been known for some time. Very surprisingly, however, aliphatic-substituted uretidiones were disclosed for the first time in German OS No. 16 70 720.
The aliphatic uretidiones produced according to the teaching of German OS No. 16 70 720, however, contain considerable amounts of the corresponding isocyanurates as "impurities", (Example 1: about 40% butyl isocyanurate; Example 2a: 49% ethyl isocyanurate, 2b: 59% ethyl isocyanurate, 2C: 79% ethyl isocyanurate).
Also, in dimerizing isophorone diisocyanate as described in German OS No. 16 70 720, no pure uretidione dimer is produced, but rather only a mixture of reaction products containing a maximum of 80% of the uretidione dimer of isophorone diisocyanate. The rest is inseparable isocyanurate derived from isophorone diisocyanate.
German OS No. 1 934 763 deals exclusively with oligomerization of IPDI using tertiary phosphines. The reaction products obtained following the teaching of this Offenlegungsschrift consist of ca. 60 parts by weight (p.b.w.) dimers (decomposable by heat), and ca. 40 p.b.w. trimers or higher oligomers of IPDI (no longer decomposable by heat). The dimer proportion can be increased to ca. 80 p.b.w. by suitable variations of the method (e.g., lower conversion). It is not possible, however, to increase the uretidione content further because the catalyst (tert. phosphine) catalyzes not only the dimerization, but also the trimerization of IPDI to the corresponding isocyanurate.
With the dimerization catalysts of the prior art according to the German OS's Nos. 1 670 720 and 1 934 763 it has not been possible until now to prepare a uretidione dimer of IPDI free of isocyanurate, i.e., a mixture of uretidione and isocyanurate or their higher oligomers was always obtained. Such an isocyanurate-free uretidione dimer of IPDI is of great interest because it makes possible, for the first time, an economical further reaction with diols, for the production of valuable starting materials for polyurethane chemistry.